As is known, hydrogen sulfide (H2S) is a highly poisonous and corrosive contaminant of natural gas and crude petroleum. While only relatively small amounts of H2S occur in crude petroleum, natural gas can contain up to 40% by volume. As a result, H2S must be removed to acceptable levels prior to delivery to the refinery or main gas distribution system. Generally, in order to meet governmental, technical and natural gas sales specifications, H2S concentrations must be at very low levels (usually less than 16 ppm).
Hydrogen sulfide is a covalent hydride structurally related to water (H2O) as oxygen and sulfur occur in the same periodic table group. However, hydrogen sulfide is weakly acidic, dissociating in aqueous solution into hydrogen cations H+ and the hydrosulfide anion HS−:H2S→HS−+H+
Hydrogen sulfide reacts with many metals cations to produce the corresponding metal sulfides.
In petroleum refineries, the normal hydrodesulfurization processes liberate sulfur from petroleum by the action of hydrogen. The resulting H2S is converted to elemental sulfur by partial combustion via the Claus process, which is a major source of elemental sulfur.
The most highly utilized processes for sweetening sour natural gas is to use amine solutions to remove the hydrogen sulfide. These processes are known simply as the ‘amine processes’, or alternatively as the Girdler process, and are used in 95 percent of North American gas sweetening operations. Generally, the sour gas is run through a tower, which contains the amine solution. This solution has an affinity for sulfur, and absorbs it much like glycol absorbing water. There are several amine solutions that are commonly used, including monoethanolamine (MEA), methyldiethanolamine (MDEA), and diethanolamine (DEA) each of which in their liquid form, will absorb sulfur compounds from natural gas as it passes through the column. The effluent gas or sweet gas is virtually free of H2S compounds. Like the process for NGL extraction and glycol dehydration, the amine solution used can be regenerated (that is, the absorbed sulfur is removed), allowing it to be reused to treat more sour gas. This technology is capital intensive and is generally more suitable for larger scale operations.
In other systems, the use of liquid scavengers within columns is also known. In these systems, sour gas and a liquid scavenger agent are introduced into a column. The scavenger reacts with sour gas within the column such that both sweet gas and “spent” scavenger are removed from the top of the column. The most common liquid scavenger is an amine-aldehyde condensate manufactured by an exothermic reaction of monoethanolamine and formaldehyde. Water and methanol are usually required to keep the formaldehyde in solution and prevent polymerization. The resulting “scavenger” product is a hexahydrotriazine, and is commonly called “triazine” in the industry. The “triazine” is typically offered in a water-based solution. In most applications, the reaction products are also water soluble, with very low toxicity characteristics making this a relatively simple system to handle. Other scavenging reagents are known to those skilled in the art.
Importantly, the scavenging reactions between triazine and H2S can be “overspent” such that the reaction products are solids. Generally, it is preferred that solid reaction products are not produced for ease of subsequent handling. Thus, most reactions are controlled to underutilize the scavenging reagent.
While the liquid scavenger system is a relatively cost effective system as a result of the relatively low capital cost of equipment, simple logistics, and simple waste treatment, the cost of scavenger reagent is relatively high. Typically, as a result of the cost of the liquid scavenger, the overall process cost of H2S removal will range from a low of $8/pound to $20/pound of H2S removed. Notwithstanding the cost of reagent, the liquid scavenger system is a preferred system for offshore gas treatment and onshore sites where there is a relatively small amount of H2S that needs to be treated.
However, there continues to be a need for a technology that improves the efficiency of utilization of scavenger reagent, such that the overall process economics can be improved.